The present invention relates generally to controllers for electric motors, and more particularly to a digital motor controller for DC brushless motors.
Controllers for electric motor have traditionally analyzed analog control loops and signals in conjunction with limited digital processing to provide both closed loop control and protection. Typically, such analog systems are comprised of a large number of discrete components. These controllers are large, costly and often unreliable.
Further, prior art motor controllers are not interchangeable. In general, a motor controller is configured for the design characteristics (e.g. resistance and inductance) of a particular motor. Design characteristics vary between different motors Power ratings also vary. Therefore, the same controller cannot be used for different motors.
Additionally, prior art motor controllers cannot provide for changing design characteristics of the motor itself. For example, during continued use, the resistance and inductance of the motor are subject to change. These changes may become increasingly more significant with increases in temperature over time as the motor is cycled through a number of operating periods during the course of its operational life. The prior art motor controllers do not allow for changes within motor design characteristics nor can they operate the motor at peak efficiency with precise control of the operational parameters.
In prior art controllers, a problem known as "shoot-through" occurs when controlling the inverter of the motor. An inverter 12 shown in FIG. 2 comprises three pairs of complementary switches 24 and 30, 26 and 32, and 28 and 34. By turning "on" selected switches, two windings can be energized by a power supply. However, if a complementary pair of switches, such as 24 and 30, are conducting at the same time, a short across the power supply occurs. As a result, the power supply is damaged.
For ideal switches, non-overlapping timing signals would prevent complementary switches from conducting at the same time. However, the switches are not ideal; they are formed by transistors, which have internal capacitances. Thus, as one transistor is charging and the other is discharging, the two transistors can conduct simultaneously. As a result, shoot-through occurs.
The limitations of prior art motor controller systems are magnified in the case where two or more motors are physically coupled to a single output in an effort to increase the total power output of the system. For these types of systems, prior art controllers are not capable of communicating with one another to provide precise synchronized motor control.